Python Questions

**Ewok** · 03-17-10, 11:17 PM

@tuples, packing, and unpacking

First, tuples are lists that you can't change once you make them. The reason they are great is because they are cheap and easy to make.

For instance
a = (1, 2)
makes an immutable list of 2 elements, the first being 1 and the second being 2

Where this gets interesting with with packing and unpacking. Python has built in syntax for copying values into tuples for you (packing) and copying values out of tuples and into variables for you (unpacking).

For instance I can do packing
a = 1, 2
which makes a tuple of (1, 2) and stores it in a

Also, I can do unpacking
b, c = (1, 2)
b is now 1, and c is no 2

Why does this matter? It saves a lot of typing

Remember that for loop example I gave with enumerate?
for index, value in enumerate(myList):

"index, value" was unpacking a tuple for you. Otherwise you would have had to do
for pair in enumerate(myList):
index = pair[0]
value = pair[1]

which is a lot more typing

The other place you'll see packing and unpacking a lot is in calling functions and returning values. The age old issue of functions is how to pass back more than one value. In C/C++/Java, you have to make a struct/class, pass it as input and fill it out.

In python, you just make a tuple-on-the-fly and return that compound value. And then the caller unpacks it on the fly

Example:
def multipleReturnValues():
a = ... some calculation
b = ... some other calculation
return a, b

firstReturnValue, secondReturnValue = multipleReturnValues()

**Ewok** · 03-17-10, 11:20 PM

@maps

In python there is a "map interface" and a dictionary type (which implements the map interface).

So at any time in python use { and } and you have an on-the-fly map

a = {} //empty map

a["asd"] = 2
a["qwe"] = 3

b = a["asd"]
(b is now 2)

You can also create them with initial values using special syntax

a = {"key1": value1, "key2": value2} (and so on, just keep adding commas and key/value pairs)

Any immutable type in python can be a map key. So strings, numbers, and tuples are all allowed (and some other things).

**Imisnew2** · 03-17-10, 11:25 PM

Thank you very much raven (and you're definately not stepping on my toes...)

And yea... I stumbled upon enumerate... just wasn't sure if it was the "right" way

@maps:

so I can say

Code:

a = {0:"Bob", 5:"Sally"}

print a[0]
print a["Bob"]

will print
'Bob'
0
?

Or does it not work bidirectional? And what if it was a 3-ary tuple?

Code:

a={0:"Bob":15, 5:"Sally":16}

is that allowed?

**Ewok** · 03-17-10, 11:29 PM

so between brackets are only keys
and it always stores or returns values

so print[0] would return "Bob"
print["Bob"] would raise KeyError since you don't have the key "Bob" in your dict

syntax for creating dictionaries is
varName = { key1: value1, key2: value2, ...}

so that 3-ary thing isn't valid

And dictionaries have methods for enumerating keys, values, and pairs of key/values (.keys(), .values(), .items() respectively)

**nsRaven** · 03-18-10, 01:27 AM

I always forget about enumerate. The problem I have with tuples though is since you can't change them after they're created, they're not practical for anything but something like enumerate and zip and using them in iterations if you don't already have a list instead.

@Imisnew2
If you're used to something like a switch/case statement you won't find that in Python. The dictionary type that Ewok explained though is very versatile and can be used as a switch/case if you need something like that.